Renal injuries are caused by damage and destruction of renal
tubules. This may occur due to a variety of different mechanisms,
ultimately all lead to hypoxia and ischemia:

Medullary ischemia
– due to hypoxic injury to the thick limb of the loop of Henle. This
leads to reduction in tubular filtration, via feedback mechanisms.

Tubular obstruction
– due to casts, which are damaged tubular cells. The tubular pressure
builds up and glomerular filtration is inhibited.

Interstitial edema:
tubular fluid leaks in between damaged cells.

Vasoconstriction:
due to release of vasoactive agents in critical illness.

Tubular obstruction,
with backflow of tubular fluid causing oliguria

It is thought that the mechanism behind oliguria is ischemic
injury to tubular cells which subsequently slough off as casts and block
the passage of fluid through the nephron. Pressure builds up in the
pre-obstruction area, and is reflected backwards towards the Bowman’s
Capsule. As the pressure increases, less and less is filtered, and
oliguria becomes pathological. When the pressure in the Capsule equals the
net filtration pressure, filtration stops and the patient becomes anuric.
One of the reasons why some experts recommend the use of diuretics is to
flush debris out of the nephrons, and allow filtration and passage of
urine. The efficacy of this technique has not been established.

When
renal physicians come to consult on our patients, they retrieve a sample
of urine and look at it under a microscope, why do they do this?

Urinary microscopy is a useful diagnostic technique for acute
renal failure in the medical setting, particularly in the early stages.
The presence of different cells or casts indicate the etiology of the
disease process. We don’t get very excited about this in ICU as the
majority of our patients develop ATN due to hypovolemia, hypotension and
sepsis. However the test is inexpensive and worth thinking about,
especially if there is a diagnostic dilemma. Table 2 summarizes the
various things that can be seem under the microscope.

Table 2
Urinalysis Findings In Acute Renal Failure

Injury

Urinalysis

Prerenal

Benign or hyaline
casts

Acute Tubular
Necrosis

Hemegranular or
epithelial cell casts

Acute Interstitial
Nephropathy

WBCs, WBC casts,
eosinophils, proteinuria

Acute
glomerulonephritis

RBCs, dysmorphic
RBCs, RBC casts, proteinuria

Postrenal

Benign ± hematuria

WBC = white blood
cell; RBC = red blood cell.

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